Optical amplifiers amplify light as it propagates within the amplifier. Optical amplifiers are used as repeaters, optical preamplifiers, and gain blocks within telecommunication systems in which optical fibers form the system backbone. Rare earth doped amplifiers, such as erbium doped fiber amplifiers, are often used in telecommunication systems. Optical amplifiers reduce the need for optical-to-electrical-to-optical conversion, in which optical signals are converted to electronic signals with a photodetector and the electronic signals are used to modulate a laser to regenerate the original optical signal. Optical-to-electrical-to-optical conversion is expensive. In wavelength division multiplexed systems using multiple wavelengths, regenerating equipment is required on a per channel or wavelength basis. Multiple signals at different wavelengths are often amplified several times as they propagate within the telecommunication system. There is a trend to transmit multiple signals each having a different wavelength within the C band of 1525.6 nm to 1562.5 nm, and the L band of 1569.4 nm to 1612.8 nm. Wavelength division multiplexing of signals provides a higher total transmission data rate for a telecommunication system.
The gain of rare earth doped amplifiers is the not flat across the wavelength range of the amplifier, but has asymmetrical peaks which reflect the fine structure of the energy levels of the rare earth ion. This is problematic when signals are distributed across the wavelength range of the amplifier. The signals and spontaneous emissions in high gain regions are amplified more than the signals and spontaneous emissions in low gain regions. Unequal amplification of the signals limits the number of wavelengths that can be transmitted through a system. To compensate, some telecommunication systems with erbium doped fiber amplifiers include a fiber with a loss profile that is the inverse of the gain spectra of the erbium doped fiber amplifiers. This flattens the gain of the system at the expense of increased signal losses, reduced signal-to-noise ratio and increased cost.
It would be desirable to have a compact optical amplifier with a flattened gain profile that would overcome the above disadvantages.